Pulverized coal combustion with opposing/cross-flow methane/air mixtures

Abstract

Pulverized coal co-firing with opposing premixed methane/air flames in a cross-flow arrangement was found to extend the coal burning limits and reduce its NOx emissions by creating a high temperature NOx-reducing zone between the resulting multiple flame envelopes. By using a primary air stream, the stability limit reached a peak value of 25.1 m/s while the exhaust NOx mole fraction got values as low as 1.2 × 10–4 due to HCN reaction with NOx generated from both the coal primary flame envelope and the premixed flames. The NOx exhaust emissions were reduced to values below 60 ppm as the gas/coal heat input ratio increased to 1.5, where a nonunity Lewis number predicted the peak NOx concentrations across the annular region surrounding the flame. The effectiveness of NOx reduction increased by employing triple flames in place of the primary air since each NOx peak generated from methane combustion was followed by a region of NOx reduction upon consuming a significant amount of the HCN released from coal. Extending the coal flame stability limit to 32.3 m/s contributed to NOx reduction via decreasing the residence time for NOx formation across the combustor exit section. Decreasing the triple flame mixture fraction gradient and keeping a higher overall equivalence ratio reduced the NOx exhaust concentrations to 50 ppm and increased the percent of heat transfer by radiation to reach a peak of 48.8%. Decreasing the particle size to 135 µm and increasing the number of opposing jets to 16 pronounced favorable aerodynamic effects by minimizing the HC and NOx emissions, respectively, to values below 0.16% and 65 ppm. Increasing the opposing jets’ diameter to separation reduced the flame length by 26.7%, while maximizing the staging height kept the NOx emissions below 40 ppm at a strain rate of 2050 s–1.